The present invention relates to a method of flash butt welding high-carbon materials and, more particularly, a method of flash butt welding modular cast iron.
In known flash butt welding methods such as that in accordance with current and force are transmitted to the parts to be joined by clamping jaws. The parts to be joined, through which DIN 1910, current flows, are heated under light contact by the formation of arcing contacts and molten material is thrown out of the end-face area by metal vapor pressure. After sufficient heating, the workpieces are welded by sudden upsetting.
The weldability of high-carbon materials, in particular materials where the carbon ranges between 2.0 to 3.9% by weight, although known for a long time, are still problematic. The reason for this lies in the hardening behavior of these materials and a tendency associated therewith to form ledeburitic and/or martensitic hardening structures which considerably restrict the constructional weldability of these material groups.
German Offenlegungschrift DE 3628246 Al, assigned to the assignee of the instant invention, discloses a method of butt welding high carbon materials by a post weld control temperature method so as to control the formation of ledeburitic and/or martensitic structures, while satisfactory welds can be obtained in relatively thin-walled workpieces, thick-walled workpieces, i.e., at wall thicknesses over 12 mm, satisfactory welded structures cannot be guaranteed, since the entire proportion of molten metal is not forced out of the interface during the upsetting operation. If the entire crater lies inside the wall thickness, the escape routes are closed during the upsetting. In thick-walled workpieces, therefore, ledeburitic, lenticular inclusions in the welding zone can generally be expected.
The inclusions arise from the sequence mechanism of the known welding operation in the following manner.
The current-carrying parts which are to be joined are moved against one another with a certain contact force and pulled apart again. When the parts to be joined are carrying current, the current is switched on and off by the parts to be joined themselves, and in the process considerable heating occurs at the contact points. This preheating cycle is repeated several times until the abutting ends have been heated in such a way that the energy supplied is sufficient to initiate the flashing process.
So-called arcing contacts occur at the locations where the heated contact points touch. In these contact areas, a very high energy density is partly introduced into the material so that rapid melting of the metal takes place at these points. However, the molten metal is not distributed homogenously over the entire contact face but collects in arcing craters created by the heated contacts. The deeper and larger these arcing craters are, the more difficult it becomes to press these liquid metal areas from the craters into the welding bead during the subsequent upsetting operation. Although a portion of this liquid metal is thrown out of the welding gap from the arcing craters as a result of the high metal vapor pressure, a considerable proportion of molten metal remains behind in the deepest points of the arcing craters, from where it can no longer be conveyed out of the welding zone by the upsetting pressure.
Accordingly, it is the principle object of the present invention to provide an improved butt-welding method for welding thick-walled, high carbon workpieces so as to produce a defect-free welded point.